1. Field of the Invention
The present invention relates to lithium-sulfur batteries, and more specifically, to lithium-sulfur batteries with good cycle life characteristics.
2. Description of the Related Art
The remarkable development of portable electronic devices has led to an increase in the demand for secondary batteries. Particularly, the secondary batteries are required to have a high energy density in relation with trends for more compact and lighter portable electronic equipment. In addition, the secondary batteries are required to be cheaper, safer, and more environmentally friendly.
The active material used in lithium-sulfur batteries is a cheap and environmentally friendly material. Further, with respect to energy density, since the energy density of a lithium-sulfur battery is anticipated to be high (i.e., that of the lithium of the negative electrode is 3,830 mAh/g and that of the sulfur of the positive electrode is 1,675 mAh/g), lithium-sulfur batteries are an attractive candidate to satisfy the above-mentioned requirements.
A conventional lithium-sulfur secondary battery uses a sulfur-based compound with a sulfur-sulfur bond as a positive active material, and a lithium metal or a carbon-based compound as a negative active material. Upon discharge (electrochemical reduction), a sulfur-sulfur bond is broken, resulting in a decrease in the oxidation number of S. Upon recharging (electrochemical oxidation), a sulfur-sulfur bond is formed, leading to an increase in the oxidation number of S. Therefore, the electrochemical energy can be stored and generated using the reduction-oxidation reactions. The above-mentioned sulfur-based compound is often referred to as an “active sulfur.”
The active sulfur has a reductive state that is not simple, and it coexists with various chemical species, successively (see E. Levillain et al., J. Electroanal. Chem. 420 (1997) 167; and E. Levillain et al., J. Electroanal. Chem. 440 (1997) 243). Overall, the reaction occurring in batteries of active sulfur is described in the following Formula 1:xLi+S⇄LixS  (Formula 1)
That is, active sulfur in lithium-sulfur batteries reacts with the lithium metal, resulting in a reduction to lithium sulfide or lithium polysulfide. The compound of the theoretical final oxidation state is S8, while the compound of the final reductive state is Li2S. When S8 is completely reduced to Li2S (100% utilization), it has a theoretical capacity of 1,675 mAh/g. Thus, capacity is higher than for any other chemical species.
Contemplated battery designs include those in which (a) all of the active sulfur dissolves in the electrolyte (one-phase positive electrode), and (b) the active sulfur is present both in a solid phase (sometimes precipitated) and in a liquid phase in the electrolyte, an example of which is disclosed in U.S. Pat. No. 6,030,720. However, regardless of which design is adopted, with cycles of charging-discharging, the active sulfur of the lithium-sulfur battery may exist in either an all-liquid phase or a solid-liquid mixed phase as shown in FIG. 1. Referring to FIG. 1, only the liquid phase exists in the decreasing curve of the discharge, while both liquid and solid phases coexist in the first and second plateau parts.
The lithium-sulfur battery is different from other types of batteries in that the solid and the liquid phases may simultaneously coexist as the positive active materials. Accordingly, in the lithium-sulfur battery, the active material may be attached to the positive electrode and dissolved in the electrolyte.
As described above, during a charging-discharging cycle, the active material is not maintained as an all-solid phase, but coexists with a liquid phase. Therefore, the active material can be electrochemically inactive (i.e., the active material may not stay within the positive electrode), but it can be detached from the positive electrode and become unavailable for further electrochemical redox reaction. Accordingly, although lithium-sulfur batteries have a high theoretical capacity, the performance thereof is not satisfactory. In addition, on the condition of the continued reduction of the lithium polysulfide, the sulfur is irreversibly precipitated on the surface of the conductive network and it is hard to oxidize again, with the difficulty greatly depending upon the composition of the electrolyte.
In order to avoid the problems and provide lithium-sulfur batteries with excellent performance, positive active materials have been proposed as follows: an organosulfur compound such as those disclosed in U.S. Pat. Nos. 4,833,048 and 4,917,974 and U.S. Pat. No. 5,162,175; DMcT-Pan (2,5-dimercapto-1,3,4-thiadiazol and polyaniline) as disclosed in Oyama et al., Nature, 373, 598-600, 1995; and a carbon-sulfur compound as disclosed in U.S. Pat. Nos. 5,441,831, 5,460,905, 5,601,947, and 5,609,720. However, the organosulfur has a low theoretical capacity and a slow reaction rate at room temperature. Further, the carbon-sulfur compound also has problems in that the theoretical capacity is low and it is not easy to reproduce the material with an identical molecular structure.
Accordingly, attempts have been made to utilize elemental sulfur (S8) as a positive active material. Since elemental sulfur has the highest theoretical capacity and is in the form of powder, an electrode can be prepared to have a high active material density per unit volume and a high capacity density, resulting in providing a positive electrode with a high capacity. In U.S. Pat. No. 5,523,179, an elemental sulfur rechargeable at room temperature is utilized in the battery system. As disclosed in this patent, the theoretical final oxidation state of active sulfur is defined as inorganic sulfur (S8).
Even when using elemental sulfur, the problems of the coexistence of solid and liquid phases of the sulfur occur. In order to overcome the problems, it has been suggested that sulfur-absorbing additives be added to a positive active material slurry to delay the detachment of the sulfur. As the sulfur-absorbing additives for this purpose, JP Laid-Open Publication No. 09-147868 discloses an active carbon fiber. U.S. Pat. No. 5,919,587 discloses techniques whereby a positive active material is embedded among transient metal chalcogenides having a highly porous, fibrous and ultra fine sponge-like structure, or that the positive active material is encapsulated therewith. Further, PCT Publication No. WO 99/33131 discloses that particulates such as carbon, silica, and aluminum oxide having a potent absorbency toward polysulfide are added. PCT Publication No. WO 99/33125 discloses that the positive electrode is encapsulated within a separator made of a microporous pseudo-boehmite layer, so as to inhibit the diffusion of soluble polysulfide. PCT Publication No. WO 99/33127 discloses that polysulfide anions are kept around a cationic polymer with a quaternary ammonium and an anionic polymer including a salt group. However, as a result of incorporating the additives to enhance the positive electrode active mass, the energy density is reduced.